expand: add prev_{desk,pagex,pagey}

Add further variables, for ascertaining what the previous desk, pagex,
pagey is for a given monitor.

doc/fvwm3/fvwm3.adoc

@@ -1661,7 +1661,7 @@ $[pointer.screen]::
 +
 This command is deprecated; use $[monitor.current] instead.
 
-$[monitor.<n>.x], $[monitor.<n>.y], $[monitor.<n>.width], $[monitor.<n>.height], $[monitor.<n>.desk], $[monitor.<n>.pagex], $[monitor.<n>.pagey] $[monitor.primary], $[monitor.current], $[monitor.output], $[monitor.count]::
+$[monitor.<n>.x], $[monitor.<n>.y], $[monitor.<n>.width], $[monitor.<n>.height], $[monitor.<n>.desk], $[monitor.<n>.pagex], $[monitor.<n>.pagey] $[monitor.primary], $[monitor.current], $[monitor.output], $[monitor.count], $[monitor.<n>.prev_desk], $[monitor.<n>.prev_pagex], $[monitor.<n>.prev_pagey]::
 	Returns information about the selected monitor. These can be nested, for
 	example: $[monitor.$[monitor.primary].width]
 +
@@ -1683,6 +1683,12 @@ monitor which has the mouse pointer.
 "pagey" returns the Y page of the referenced monitor.
 +
 "primary" is the name of the output set as primary via xrandr(1).
++
+"prev_desk" returns the previous desk on the referenced monitor.
++
+"prev_pagex" returns the previous X page on the referenced monitor.
++
+"prev_pagey" retuns the previous Y page on the referenced monitor.
 
 $[screen]::
 	The screen number fvwm is running on. Useful for setups with multiple

fvwm/expand.c

@@ -594,6 +594,26 @@ static signed int expand_vars_extended(
 					monitor_get_all_heights());
 				goto GOT_STRING;
 			}
+
+			if (strcmp(rest, "prev_desk") == 0) {
+				is_numeric = True;
+				val = m->virtual_scr.prev_desk_and_page_desk;
+				goto GOT_STRING;
+			}
+
+			if (strcmp(rest, "prev_pagex") == 0) {
+				is_numeric = True;
+				val = (int)(m->virtual_scr.prev_page_x /
+					monitor_get_all_widths());
+				goto GOT_STRING;
+			}
+
+			if (strcmp(rest, "prev_pagey") == 0) {
+				is_numeric = True;
+				val = (int)(m->virtual_scr.prev_page_y /
+					monitor_get_all_heights());
+				goto GOT_STRING;
+			}
 		}
 		break;
 	}